1. Field of the Invention
The present invention relates generally to scraper apparatus used in scraping filter cake from a moving filter medium. The invention, more specifically, relates to scraper apparatus or devices used with disc filters, drum filters and belt filters to dislodge filter cake from the filter medium as the filter medium moves past the scraper means.
2. State of the Art
There are various types of filters that utilize a moving filter medium. Disc filters utilize a rotating disc that revolves so that during a portion of its movement it is submerged in a liquor or slurry that is to be filtered. A drum filter is similar to a disc filter in that it revolves so that a portion of the drum is submerged in a liquor or slurry to be filtered. Common to both disc and drum filters, filter cake is removed from the filter medium during the portion of its movement when it is not submerged in the liquor or slurry being filtered.
Another common filter apparatus using a moving filter medium is a belt filter. Generally, the belt filter moves over a suction box and slurry is deposited on the filter, with mother liquor being pulled from the filtrate through the belt filter into the suction box. The belt filter must have filter cake removed from the filter medium during the portion of its travel from the tail end of the suction box back to the head end of the suction box. A scraper is commonly used on disc, drum and belt filters to aid in the dislodging and removal of filter cake from the filter medium as the filter medium moves past the scraper.
Continuous rotary filters operate by having a filter medium formed in the shape of a disc, with the disc rotating through a pool of filterable slurry. The inner side of the portion of the filter medium submerged in the pool is subjected to a reduced pressure for drawing liquid through the filter medium and depositing solids on the outside of the filter medium as a filter cake. Normally, such filters operate with about half of the filter medium submerged in the slurry and the remainder in the atmosphere. During travel of the filter cake through the atmosphere, the filter cake is subjected to washing and drying actions and is then subjected to a cake removal action. Cake removal is normally achieved by blowing air through the filter medium from the inner side as to blow the cake from the outside of the filter medium. To aid removal of the cake from the filter medium, a scraper is positioned to scrape the cake from the filter medium while it is under the influence of the blow-back air pressure.
Disc filters are constructed as a series of relatively thin, truncated wedges mounted in a annular arrangement to form a disc. Generally, each sector is covered by a bag made of filter medium, and such bags are tied at their lower ends to the sector bells and held at their outer ends. Since the filter medium or filter cloth bag is normally only secured at its outer and inner ends, blow-back pressure subjected internally of the bag bulges outwardly its sides. The greatest displacement of the bag on such bulging, of course, is along the middle portion of the sector. When such a bulging filter medium comes in contact with the scraper, the filter medium drags across the scraper causing wear.
A number of attempts have been made to overcome the effects of this filter medium bulging. In one such method, wires have been wrapped around the sector to hold the filter medium bag tightly on the filter sector. Since there are a substantial number of sectors in each disc and a substantial number of discs in each filter, the expense of wrapping wire around each sector greatly exceeds the savings achieved by extended bag life. In another attempt, a rubber scraper blade was stretched across the radial span of the bag. However, unless such a rubber blade is supported along its length, it curls downwardly under the influence of the bulging bag and is thus inoperative. If, on the other hand, such a rubber blade is supported by sufficient lengthwise reinforcing to stop the curling, it is too stiff and it cannot contour along the bulge line of the filter bag.
Most commercial disc filters utilize scrapers which are straight, fixed blades mounted with a spring mounting arrangement to permit some movement of the blade against the inflated bag. Because the blade is rigid, only the center portion of the bag is scraped, and the end portions which contour away from the bulged-out middle portion do not contact the scraper blade and are thus not cleared. Such an arrangement under optimum working conditions cleans less than about 75% of the bag. If the blade tension against the bag is increased, the bag overhangs the blade greatly increasing the drag on the bag, and greatly increasing wear.
The problem of dislodging and removing filter cake is also present when using a drum filter or belt filter. Drum filters and belt filters can be subject to the filter medium bulging as described previously with respect to disc filters. Filters of all types have presented a problem in effectively dislodging and removing filter cake from the filter medium without undue wear of the filter medium. In the past, it has been the universal practice to scrape the filter medium with a chisel edged device in which the chisel edge makes sharp, abrupt contact with the filter medium. It was thought that it was necessary to make a sharp contact to efficiently dislodge the filter cake from the filter medium. Thus, all scrapers have been made of blades that slant with respect to the surface of the filter medium so that only an edge of the scraper contacts the filter medium.